A common problem with high intensity discharge (HID) lamps is that, after an extended period of operation, the arc becomes unstable, causing the light output from the lamp to flicker. Such arc instability is believed to be controlled by a number of factors including: arc tube shape; composition of the metal halide fill; amount of free halide in the arc tube during lamp operation; arc tube operating power; and the temperature of a liquid pool of metal halide that forms in the lamp during lamp operation.
One approach to avoiding arc instability involves reducing the rate of free halide buildup in the arc tube. For example, as described in commonly assigned U.S. Pat. No. 5,507,751 of Witting et.al., issued Oct. 15, 1991 and incorporated by reference herein, a protective silicon coating of sufficient thickness may be applied to the inner surface of the arc tube to avoid a substantial loss of the metallic component of the fill and hence a corresponding buildup of free halide. By thus avoiding arc instability, the useful life of the arc tube is extended.
Although reducing the rate of free halide buildup, such as by the method of Witting et.al., U.S. Pat. No. 5,057,751 cited hereinabove, is effective in extending the useful life of the arc tube of an HID lamp, it is desirable to extend that life even further. Accordingly, it is an object of the present invention to avoid arc instability in an HID lamp in order to extend the useful life thereof.